SECTION 17-4 Equilibrium and Hearing
Hearing
The special sense of hearing and equilibrium are provided by the inner ear which is a receptor complex located in the peterous part of the temporal bone of the skull
Equilibrium sensations inform us of the position of the head in space
Hearing enables us to detect and interpret sound waves
The basic mechanisms for both senses are hair cells, a mechanical sensors
The anatomy of the ear
Three anatomical regions
External ear
Middle ear
Inner ear
Both equilibrium and hearing are provided by receptors of the inner ear
Anatomy of the ear – External Ear: visible portion, collects and directs sound toward the middle ear: compostion
Auricle or pinnae surrounds the ear
External acoustic meatus ends on tympanic membrane
External ear
Includes the fleshy and cartilaginous auricle
This surrounds the external acoustic canal or ear canal
This is the passage way that ends on the tympanic membrane
Protective features found here in the form of ceruminous glands which produce cerumen
Figure 17.20 The Anatomy of the Ear
Middle ear
Communicates with pharynx via pharyngotympanic membrane
Middle ear encloses and protects the auditory ossicles
Figure 17.21 The Middle Ear
Middle Ear
Also called the tympanic cavity
It is separated from the external acoustic canal by the tympanic membrane
Communicates with the nasopharnyx through the auditory tube and the mastoid air cells
Also called the pharyngotympanic tube which permits equalization of air
Auditory Ossicles
Hammer
Anvil
Stirrup
What are the articulations?
Malleus attaches to the tympanic membrane
The stapes articulates on the oval window
How is sound carried?
It is the articulations of the hammer on the vibrating tympanic membrane that is passed to the stapes which moves up and down on the oval window
It is really a rocking motion on the stapes
This is a level design that amplifies sound because the tympanic membrane is heavier then the membrane of the oval window
What is the job of the inner ear?
The sense of equilibrium and hearing are provided by receptors of the inner ear
Remember that these receptors lie within a collection of fluid filled chambers known as the membranous labyrinth which is filled with an electrolytic soln called endolymph
Inner ear: bony labyrinth: function
Bony labyrinth surrounds and protects membranous labyrinth
Between the bony and membranous labyrinth is found perilymph (CSF)
What are the divisions of the bony labyrinth?
Vestibule: pair of membraneous sacs
Saccule
uticle
Semicircular canals
stimulated by rotation of the head
Cochlea
Provide the sense of hearing
Figure 17.22 The Inner Ear
Components of the inner ear: quick review: are what?
Vestibule contains the utricle and saccule
Semicircular canals contain the semicircular ducts
Cochlea contains the cochlear duct
Windows: two types: functions:
Round window separates the perilymph from the air spaces of the middle ear
Oval window connected to the base of the stapes
Basic receptors of inner ear are hair cells
Provide information about the direction and strength of stimuli
Receptors of the inner ear
These sensory receptors are called hair cells
These cells are surrounded by supporting cells and are monitored by sensory afferent fibers
The hair like structures have two components
Stereocilia: 80 – 100 present
Kinocilium: single large cilia
Only move when external forces push against them
What kind of information will these cilia provide?
Direction and strength of the mechanical stimulation and response varies depending on the location of the cilia
Types of stimulation can include:
Gravity or acceleration in the vestibule
Rotation in the semicircular canal
Sound in the cochlea
How is equilibrium information provided?
Provided by receptors of the vestibular complex
The information provided is based on rotational movements of the head
Thus the saccule and the utricle convey information with respect to gravity
They are stimulated by sudden acceleration (stop or start)
The semicircular ducts
Thus the sensory receptors are quiet during non movement
What is this movement?
The kinocilia and the sterocilia are embeded in the cupula
Cupula floats on the endolymph
The movement of ones head distorts the receptor processes
Movement is based on direction
When there is no further movement, the cupula returns to the rest position
Thus there is analysis of motion in three planes
What is the job of Utricle and Saccule?
Both provide information about equilibrium whether or not the body is stationary or moving
Equilibrium: The whole structure è otolith
Anterior, posterior and lateral semicircular ducts are continuous with the utricle
Each duct contains an ampulla with a gelatinous cupula and associated kinocilia and sterocilia (review)
Saccule and utricle connected by a passageway continuous with the endolymphatic duct
Terminates in the endolymphatic sac
Saccule and utricle have hair cells clustered in an oval structure called the maculae
Cilia contact the statoconia ( calcium carbonate crystals)
Figure 17.23 The Vestibular Complex
Figure 17.23 The Vestibular Complex
Figure 17.23 The Vestibular Complex
Vestibular neural pathway: How is monitoring achieved?
Hairs of the vestribular and semicircular ducts are monitored by sensory neurons located in the vestibular ganglia
Axons form the vestibular branch of the vestibular cocohlear nerve (VIII)
Synapses within the vestibular nuclei between the pons and the medulla oblongata
Job functions; 4 of them
Integrating sensory information about balance and equilibrium that arrives from both sides of the head
Relay information from the vestibular complex to the cerebellum
Relay information from the vestibular complexd to the cerebral cortex for a conscious position of position of head
Send motor commands to nuclei in brain stem and spinal cord
What kind of information is sent?
Reflexive motor commands that are issued are distributed to motor nuclei for cranial nerves III, IV, VI, and XI
Descend down the vestibularspinal tracts
Adjust muscle tone
Figure 17.24 Pathways for Equilibrium Sensation
Hearing
Cochlear duct lies between the vestibular duct and the tympanic duct
Hair cells of the cochlear duct lie within the Organ of Corti
Intensity is the energy content of a sound
Measured in decibels
Figure 17.25 The Cochlea
Figure 17.26 The Organ Of Corti
Hearing
The receptors of the cochlear duct provide the sense of hearing that enables us to detect soft sounds
Hair cells responsible for picking up this auditory sound
Location prevents them from responding to any other stimuli
Whole process is based on pressure waves
This is the fluctuations of perilymph which determine the frequency and intensity
Pathway of sound
Sound waves travel toward tympanic membrane, which vibrates
Auditory ossicles conduct the vibration into the inner ear
Tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive
Movement at the oval window applies pressure to the perilymph of the cochlear duct
Pressure waves distort basilar membrane
Hair cells of the Organ of Corti are pushed against the tectoral membrane
It is the distortion of the basiliar membrane pressing on the tectorial membrane that results in the generation of an action potential in the receptors
Figure 17.28 Sound and Hearing
Figure 17.29 Sound and Hearing
Neural pathway; location of the nerve fibers
Sensory neurons of hearing are located in the spiral ganglion of the cochlea
Afferent fibers form the cochlear branch of cranial nerve VIII
Synapse at the cochlear nucleus
The steps:
Sound waves arrive at the tympanic membrane
Tympanic membrane causes displacement of auditory ossciles
Stapes moves against the oval window
Pressure waves distort the basilar membrane
Vibration of the basilar membrane
Relay information along the afferent branch of the Vestibularcodhlear nerve VIII to the cochlear nucleus then crosses to opposite side of the brain to the inferior colliculus
Then to the thalamus and finally to the auditory cortex of the temporal lobe
You should now be familiar with:
The sensory organs of smell, and the olfactory pathways in the brain.
The accessory and internal structures of the eye, and their functions.
How light stimulates the production of nerve impulses, and the visual pathways.
The structures of the external and middle ear and how they function.
The parts of the inner ear and their roles in equilibrium and hearing.
The pathways for the sensations of equilibrium and hearing.
